The present invention relates to a method for the manufacture of a high-tensile-strength light waveguide with a plastic layer provided on the optical fiber thereof, by applying a hardenable polymer precursor stage to the optical fiber immediately after the fiber-drawing process.
Light waveguides consist essentially of optical fibers which have an index-of-refraction profile such that incident light is guided in them, following all curves. They can, therefore, serve as a transmission medium. For the optical transmission of information, light waveguides are required which, in addition to low optical losses, also exhibit high tensile strength. The light waveguides presently used for the transmission of information therefore consist of glass, a material with a theoretically very high tensile strength based on the bonding strength of the glass components. To obtain this high tensile strength in the glass fibers, the optical fibers are jacketed immediately after the drawing process with a protective layer of plastic. The optimum layer thickness of the plastic film depends on the mechanical properties of the latter such as the modulus of elasticity and the hardness, and is in general 10 to 100 .mu.m. According to experience, layer thicknesses of less than 10 .mu.m lead to damage to the glass surface, which reduces the tensile strength of the fiber drastically.
After the coating, the fiber is jacketed in an extrusion process with loosely or firmly adhering plastic tubing in order to protect the optical fibers against mechanical stresses. This cabling process assumes that the tensile strength of the fiber is high enough after the application of the first protective plastic film, and that the softening point of the plastic layer is above the temperature occurring in the extruder head, such that the optical fiber does not stick to the plastic tubing in the extrusion process.
In the coating of optical fibers with plastic in order to obtain high tensile strength and for protection against mechanical damage, the bare fiber passes immediately after the drawing process through one or more coating devices, which are followed by respective drying sections (ovens) for hardening the plastic. For this varnishing technique, non-reactive coating materials, i.e., soluble polymers such as cellulose acetate, polyvinylidene fluoride or polyesterimide can be used (see "Siemens Forsch.-u.Entwickl.-Ber." Vol. 6, 1977, No. 5, Pages 314 to 319). It is a disadvantage of this technique that typically only very thin layers (about 5 .mu.m) can be applied uniformly in one coating operation. With thicker layers, the film of liquid is broken apart when the fiber enters the hardening oven due to the sudden reduction of the viscosity, and the material flows along the fiber before hardening sets in, resulting in nonuniform coating (so-called "pearl-string effect"). For sufficiently thick protective layers, two or more coating operations are, therefore, required.
With reactive coating materials such as epoxy resins (see "Siemens Forsch.-u.Entwickl.-Ber.," Vol. 7, 1978, No. 3, Pages 158 to 165), which up to now have been used without solvent, only thin layers can be applied uniformly in one coating operation with thermal hardening. While a thickness of about 30 .mu.m per coating cycle can be reached if thermally cross-linkable polysiloxanes are used, such a protective layer is very soft and easily damaged. For this reason, a second coating process is generally necessary.
As an alternative to the thermally hardening systems mentioned, UV-cross-linkable epoxy acrylates have also been used recently as coating material (U.S. Pat. No. 4,099,837). There, a polymerization product of a mixture of polymer precursor stages, which are obtained by reaction of acrylic acid with a mixture of an aliphatic and an aromatic diglycidyl ether, is located on the glass fiber. In such systems, layer thicknesses of 20 to 50 .mu.m are supposed to be obtained in one coating operation; however, it is unclear at this time what the aging behavior of the materials mentioned and their influence on the static fatigue of the optical fibers are.